Can symptoms of pulmonary embolism (chest tightness, heart palpitations) come and go, or will they be constant if it's a dangerous pe clot?

variable: No predictable rules with pulmonary embolism. Sometimes the only symptom is a fast heart rate. If you are on birth control pills and having chest tightness + palpatations, you should consdier a pe and have the proper testing.

Yes: Many causes however, so exam and testing is required to be definitive. Small recurrent emboli are known and can be intermittent and variable, and eventually lethal.